There is a growing demand for devices that are able to generate microscopic-sized liquid droplets, and in many cases to print onto solid surfaces. As a biomedical example, microarray technology has been developed to detect and analyze proteins and/or nucleic acid material (e.g., DNA or RNA) within a sample. These devices utilize highly parallel hybridization assays using an array of testing sites with deposited samples on a chip or slide. This technology has been useful in gathering information for genetic screening and expression analysis, as well as the detection of single nucleotide polymorphisms (SNPs). In addition, microarray technology can be utilized in other areas such as pharmacology research, infectious and genenomic disease detection, cancer diagnosis, and proteonomic research.
These microarray devices, however, require the formation of high-density hybridization sites or spots on a solid surface. The high-density array of test sites is generally formed using either photolithographic patterning techniques, mechanical microspotting, or inkjet like printing. The photolithographic method fabricates microarrays through on-chip chemical synthesis of DNA molecules using spatially directed exposure of light to selectively de-protect regions of the substrate. Affymetrix, Inc. of Santa Clara, Calif., for example, has developed this approach. While high-density test sites may be created using this method, there are significant manufacturing costs due to the use of light blocking masks and related lithographic equipment. This process, while suitable for large-scale production, is simply too expensive for small or intermediate scale productions.
In a second method, mechanical micro spotting is used to print small amounts of solutions onto solid surfaces such as glass, silicon, or plastic substrates to form a testing array. The mechanical micro spotting technique utilizes multiple fountain pen-like pins that leave droplets on the solid surface after contact is made between the pen “tip” and the surface. This method is generally simple and inexpensive for making a small number of microarray chips. Unfortunately, after repeated use, the tip of the pin (which is typically stainless steel) tends to deform plastically, thereby resulting in test sites having inconsistent spot size and shapes.
In yet a third method, inkjet printing techniques are employed that forcibly eject fluid droplets from a printhead structure. The ejected droplets fly through the air and land on the substrate. While inkjet technology is mature and widely used in the case of traditional inkjet printers (used in the home and in business), the same technology cannot be directly translated into microarray applications. For example, in microarray applications, the droplets contain specific quantities of biological material (e.g., nucleic acids). Unfortunately, the number of samples deposited per area on the surface (i.e. average sample density on a spot) may vary widely because of splashing or spreading of droplets on the printing surface which could result in inconsistent hybridization data being generated.
More recently, a technique of “soft printing” has been developed to transfer droplets containing a biological material from one surface to another. Soft printing involves transfer of one or more droplets through liquid-solid contact. This method avoids the limitation described above with respect to pin-based (mechanical) printing and inkjet-based printing. While consistent volumes of droplets can be generated with soft printing print heads, this consistency was found to be compromised after printing because the printing action leaves a small, but noticeable residual volume behind in the nozzle. In addition, the residual volume could be a potential source of cross-contamination for subsequent printing processes.
There thus is a need for a print head device teat promotes the complete or substantially complete transfer of discrete drops from a nozzle. In this regard, no residual droplet material remains in the nozzle after printing. Such a device would enable the printing of different sample droplets through a single nozzle, enabling a flexible and compact system. In addition, such a device would improve printing efficiency since little or no cleaning steps would be required to avoid cross-contamination among printed spots.